Touch sensitive panels overlaying a display terminal screen are well known. The terminal responds to a touch at any location within a work area on the touch sensitive panel and identifies the location of the touched section. Such a system typically employs a capacitive sensing technique. Capacitive sensing systems typically have a controller which continually transmits a scanning signal across the touch panel and samples the work area until a touch is detected. When an area on the touch sensitive panel is touched by a user, the capacitance of the user's body is added to the circuit. The controller senses the resulting change in capacitance and identifies the specific situs touched.
As the reliability of touch sensitive input devices has improved, and diversity of application software expanded, touch sensitive devices, as a method of input, should be expected to be more widely used since no training is required to learn how to use such input devices. Thus, the computer user is not required to be a proficient typist or to know how to use an input device or computer. The hardware configuration is one that may be used without an alternate input device, such as a keyboard, offering flexibility in terms of space and portability. Applications using this sort of touch sensitive input device already include, and certainly are not limited to, diagnostic analysis, retail point of sale terminals, simulated aircraft flight controls, locator or informational displays, interactive educational systems, machine control, or keyboardless entry systems.
Capacitive touch screen devices are sensitive to changes caused by component temperature, component age, and stray capacitances. The devices must therefore be calibrated before initial use and periodically thereafter in order to maintain accuracy. The calibration process typically used with prior art technology involves displaying a mark in the active video area of the display screen, asking the user to touch the calibration mark as viewed on the overlaying touch sensitive panel, and then having an internal touch controller read and store touch position information. This is then repeated typically 2 to 8 times. The results of the calibration measurements are quantified by the touch controller as deviations from a standard, or otherwise as a set of correction factors, and are stored within the touch controller circuitry. After calibration, the touch controller automatically applies the correction factors to each new touch so that the coordinates sent to the host system are as accurate as can possibly be made. However, this process has the undesired step of external operator intervention in order to perform calibration.
A need thus exists for an apparatus which provides a means to automatically calibrate a touch sensitive panel used in conjunction with a fixed element display system.